The present invention represents an improvement over sewer pipe gaskets embedded in precasted wall openings or ports of underground concrete structures, such as in manholes, or embedded in concrete walls poured at construction sites. A common concern in sewer construction is the potential for leakage around a gasket joining an underground pipe and a wall opening after the connection and backfilling have been completed. ASTM Standards have addressed this concern by establishing certain performance requirements for flexible watertight joints in circular concrete sewer and culvert pipe and in precast manhole sections using rubber gaskets for sealing the joints. In particular, ASTM Standard Specification C443-94 requires there be no leakage under a hydrostatic pressure of 10 psi (23 feet head) for a period of 10 minutes with the pipe deflected in shear to create a position one-half inch wider than the assembled position on one side of the outside perimeter. Under the same pressure conditions, ASTM Standard Specification C923-84 requires there be no leakage with the pipe in axial deflection of at least seven degrees in any direction, and under a specified loading in shear. Although ASTM Standards are met under the prescribed test conditions, during installation or after extended usage the pipe may become out-of round, shift within the opening, or bottom out due to backfilling, ground settling, overhead traffic and similar transverse loads on the pipe. The clearance between the pipe and one side of the pipe narrows and compresses the gasket while the clearance on the opposite side widens allowing the gasket to relax thereby weakening its ability to withstand leakage.
Various gasket configurations have been used or proposed to eliminate the problem but with only limited success. Their designs do not maintain a good seal under extreme conditions often occurring in certain sewer installations. For example, the resilient gasket described in U.S. Pat. No. 4,333,662 consists of an annular base partially embedded in the wall opening of a precast manhole. A hollow O-ring extending from the base by a continuous radial web receives the pipe around its periphery. Any load on the pipe which deflects the pipe transversely will compress the gasket on the one side between the pipe and the wall and displace air confined in the O-ring to the other side where it can expand the O-ring into tighter contact between the pipe and the wall to maintain the seal. Other patents similarly disclose gaskets with hollow O-rings extending from embedded bases in which the ring can be injected with a material under sufficient pressure to expand it into closer contact with the pipe. Instead of a hollow ring, another known gasket configuration employs a solid O-ring extending from an embedded base by a resilient pleat-like web. Obviously, the amount the pipe can deflect or bottom out and still enable such gaskets to maintain a positive seal around the side of the pipe of widened clearance is limited.
Gaskets as above-described are embedded in the perimeter of wall openings at the time the concrete sections are poured. Typically, a pair of dish-like gasket retainers, such as shown in U.S. Pat. No. 4,333,662, are arranged back-to-back and gripped against opposite sides of the gasket by bolts or lugs extending through the retainers. With the retainer held between opposite sides of the concrete form where an opening is to be located, concrete is poured into the form and around the gasket and retainer thereby embedding the base of the gasket around the interior surface of the wall opening.
These fixtures are used primarily to embed gaskets in sewer structures precasted at a fabricator's facilities. However there is a need for a fixture which makes it possible for the gasket to be easily embedded in a concrete wall poured at a construction site.